This invention relates to a current amplifier comprising an input terminal for receiving an input current, an output terminal for supplying an output current, a first transistor having a base-emitter junction coupled to the input terminal, and a second transistor having a collector coupled to the output terminal and having a base-emitter junction which, in series with a voltage source, is arranged in parallel with the base-emitter junction of the first transistor.
Such a current amplifier is used as an attenuating current mirror in which the ratio between the output current and the input current is very small. Such an amplifier is capable of realising attenuation factors of ten times up to a million times.
A current amplifier of the type defined in the opening paragraph is known from U.S. Pat. No. 3,829,789. In this known amplifier the ratio between the output current and the input current is determined by the value of the voltage of a voltage source arranged in series with the emitter of the second transistor. In this arrangement the first and the second transistor are of the same conductivity type. Although only transistors of the NPN conductivity type are shown, the circuits could also be constructed by means of transistors of the PNP conductivity type. If the first and the second transistor are of the NPN conductivity type the input current and the output current will both flow into the current amplifier. The current amplifier then behaves as a current-sinking attenuating current mirror. If the first and the second transistor are of the PNP conductivity type the input and output currents will both flow out of the current amplifier so that the amplifier now behaves as a current-sourcing attenuating current mirror.
In integrated circuits PNP transistors present a substantially higher series resistance than NPN transistors of comparable dimensions. Even for comparatively small currents this causes the voltage drop across a PNP diode or a diode-connected PNP transistor to increase to a greater extent than anticipated on the basis of the well-known diode equation which defines the relationship between the diode current and the diode voltage. In the PNP version of the known current amplifier the series resistance has an adverse effect on the operation of the circuit. Since the voltage drop across the diode-connected first transistor increases excessively as the input current increases, the output current will increase to a more than proportional extent. Consequently, the current transfer of the amplifier will be non-linear. This problem does not occur or hardly occurs, in the NPN version of the known current amplifier. The current transfer is substantially linear for comparable input and output currents. However, since the NPN version can only sink input and output currents its use is limited.